Central tire inflation system rotary air union

ABSTRACT

A central tire inflation system for maintaining tire inflation on a vehicle is provided. The central tire inflation system includes a rotary union assembly having a housing that is coupled to and rotates with the hubcap. The housing is rotationally coupled to a stationary shaft. A flexible torque transfer shaft is connected to the stationary shaft on one end and to an axle plug on the other end. The axle plug has a hub forming a bore where the hub is movable on a web of material. The flexible torque transfer shaft and hub have cooperative non-circular shapes to resist rotation. An air vent is provided in the rotary union assembly that is isolated from the lubrication area, which inhibits the pressurized air from the central tire inflation system from pressurizing the lubrication area.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application claims priority to U.S. Provisional PatentApplication Ser. No. 61/595,581, filed Feb. 6, 2012, titled the same andincorporated herein by reference as if set out in full.

FIELD

The technology of the present application relates to central tireinflation systems (hereinafter “CTIS”) used to maintain the tirepressure of a vehicle in operation. More specifically, the technology ofthe present application provides rotary air unions used in supplyingpressurized air to the rotating tires of trailers with hollow, non-driveaxles.

BACKGROUND

This disclosure incorporates herein by reference, in its entirety, U.S.Pat. No. 7,207,365, entitled “Central Tire Inflation System Rotary AirUnion.”

Tire pressure maintenance is and has always been important to properoperation of a vehicle. The trucking industry has promoted this subjectas a means to increase tire life and fuel economy, while reducingdowntime and maintenance costs due to flat tires or the like. A tireblowout on the road can be unsafe for a number of different reasons,including difficulty controlling the vehicle as well as debris left onthe roadway that can be hazardous to other drivers. Proper tire pressurecan decrease the possibility of tire failures and may increase safeoperation for the vehicle driver and other drivers on the road.

Various CTISs have been designed and are the subject of numerous patentsthat accomplish the objective of tire pressure maintenance. The mostcommon systems in the heavy truck industry are designed for trailers.Heavy trucks typically include a tractor and a trailer. Often, thetrailer axles are hollow with axle ends that commonly have a throughbore. The hollow axle provides a conduit to supply air pressure to thewheel end. Delivery of air via a hose is more challenging for steeraxles and drive axles in part due to typically solid spindles on steeraxles and solid rotating shafts inside drive axles. In all cases, steer,driven, and trailer, the wheel end assembly includes a lubrication areabetween the axle and the wheel. This may require plugging the throughbore in the axle end and covering the end of the axle with a hubcapattached to the wheel. The wheel is supported on the axle end by wheelbearings. The bearings require lubrication and the integrity of thelubrication area is essential in maintaining the operability and life ofthe wheel end assembly. In order to provide pressurized air to therotating tires, the CTIS may include a rotary union in the same generallocation as the bearings. In most cases, the CTIS is in or adjacent tothe lubrication area between the stationary axle and the wheel. The CTISshould not allow pressurized air into the lubrication area. Pressurizedair in the lubrication area may cause the lubricating oil to be forcedpast the wheel seals by air pressure leading to bearing failure andconsequently catastrophic wheel failure.

Two methods are commonly used to supply pressurized air to the end of atrailer axle, pressurize the axle itself, or use a smaller conduit, suchas an air hose, inserted within the axle. In either case, the airpressure at the end of the axle is then communicated from the stationaryaxle to the rotating wheel by the use of a rotary union.

The rotary air union assembly in combination with a regulated airpressure source functions to deliver air from the stationary axle to oneor more rotating tires. The regulated air pressure source uses vehicleair pressure typically supplied by the tractor's air compressor or thetrailer's air pressure tanks, which may also provide a reservoir of air(or other gas) for operation of the pneumatic brakes. The regulated airpressure source for the CTIS may include a filter, a regulator, airtubing, and fittings. A flow or pressure sensor may be included to senseair supply to the tires. Also, an isolation valve may be provided toisolate the CTIS from the regulated air pressure source. If the CTISincludes a sensor, generally, a light is included to alert the driver oroperator that the CTIS is supplying air to the tires, which often needsto be observed by the driver by use of the rearview mirror.

The primary seal within the rotary air union also takes many forms; aspring loaded face seal, o-ring seals, u-cup seals, or packing materialseals. The primary seal is a key element of the rotary union; however,even in the best conditions, air can escape from the primary seal andpressurize the lubrication area. Vents to atmosphere have been disclosedincluding the vent chamber, vent passageway, and check valve, such asdescribed in the above-referenced U.S. Pat. No. 7,207,365.

A further function of a typical rotary union is to supply air from thestationary axle to the rotating wheel and tire when misalignment betweenthe two is present. Varied approaches to this challenge have beendisclosed. In each case, the associated seals and connections are torquecarrying elements between or within the stationary and rotatingcomponents of the rotary union. Prior solutions drive torque througho-ring seals, conduits, threaded connections, and fittings, for example.In the referenced patent, U.S. Pat. No. 7,207,365, a coupling is claimedto limit the application of rotational torque on the flexible conduitthat is supplying air to the rotary union.

Tire pressure monitoring systems are available to sense, report, andoptionally record the current status and pressure history of one or moretires. An example is the BatRF® system provided by Stemco LP ofLongview, Tex. Various aspects of the present disclosure provide theability to integrate a monitoring system into the maintenance system.

Thus, against the above background, it would be desirable to provide animproved device to couple the rotating and non-rotating parts of a CTISsystem to reduce the effect of torque on the various components.

BRIEF DESCRIPTION OF THE DRAWINGS

The technology of the present application will be further explained withreference to the drawing figures referenced below, wherein likestructures may be referred to by like numerals throughout the severalviews thereof.

FIG. 1 is a perspective view of a rotary air union assembly andmonitoring system consistent with the technology of the presentapplication;

FIG. 2 is an exploded view of the rotary air union assembly of FIG. 1;

FIG. 3 is a cross-sectional view of the rotary air union assembly ofFIG. 1;

FIG. 4 is another cross-sectional view of the rotary air union assemblyof FIG. 1;

FIG. 5 is a cross-sectional view of the flexible torque transfer shaftof FIG. 3;

FIG. 6 is a perspective view of the flexible torque transfer shaft ofFIG. 5;

FIG. 7 is a perspective view of the stationary shaft of FIG. 3;

FIG. 8 is a perspective view and cross-sectional view of the axle plugof FIG. 3;

FIG. 9 is a sub-assembly of the rotary air union assembly and flexibletorque transfer shaft; and

FIG. 10 is a cross-sectional view of the rotary air union assembly andflexible torque transfer shaft coupled to an axle.

While the above-identified drawing figures set forth one exemplaryembodiment, other embodiments of the present invention are alsocontemplated, as noted throughout. The technology of the presentapplication is described by way of representative examples and shouldnot be construed as limiting. Numerous other modifications andembodiments within the spirit and scope of the technology of the presentapplication are incorporated herein.

DETAILED DESCRIPTION

The technology described within the present application providesexamples, and is not intended to limit the scope, applicability orconfiguration of the invention. Rather, the ensuing description willprovide those skilled in the art with an enabling description forimplementing embodiments of the technology. Various changes may be madein the function and arrangement of elements without departing from thespirit and the scope of the technology described herein.

Thus, various embodiments may omit, substitute, or add components asappropriate. For example, the technology may be described asincorporating a flexible tube. The single flexible tube may be replacedwith metallic tubes, bores, or the like as appropriate. Additionally,methods of manufacturing and/or assembly may be disclosed, but themethods disclosed may be performed in an order different than thatdescribed, and that various steps may be added, omitted or combined.Also, aspects and elements described with respect to certain embodimentsmay be combined in various other embodiments. It should also beappreciated that the following systems, methods, and devices mayindividually or collectively be components of a larger system.

FIG. 1 illustrates a Rotary Air Union (RAU) Assembly 100 and TirePressure Monitoring System 150 according to an exemplary embodiment ofthe technology. The RAU Assembly 100 of FIG. 1 is mounted between thehubcap 102 and the sight window 104 (which may include a vent 105). Theconstruction of the hubcap 102 and the sight window 104 (with or withoutthe vent 105) are generally known in the art and will not be furtherexplained herein except where necessary to provide context of detailsabout the technology of the present application. One exemplary sightwindow 104 includes the SENTINEL®, which is available from STEMCO LPlocated in Longview, Tex. The SENTINEL provides a vent 105 for thelubrication area. The RAU assembly 100 of this exemplary embodiment iscoupled between the hubcap 102 and the sight window 104 and provides fora fluid path from the hubcap 102 to the sight window 104 as will befurther described below. Notably, the fluid path of the RAU assembly 100allows for the sight window 104 and the vent assembly 105 to function ina conventional manner without modification.

As will become apparent on reading the present application, the RAUassembly 100 of FIG. 1 is “self-contained”. In other words, the assemblymay be sub-assembled (or preassembled) and later installed between thehubcap 102 and the sight window 104. The RAU assembly 100 may bedesigned to use existing seals, such as, for example, the hubcap seal101 (shown better in FIG. 2) between the hubcap 102 and sight windowassembly 104 to maintain a proven assembly in all vehicle applications.

With reference to FIGS. 1 and 2, the RAU assembly 100 includes a housing162. The housing 162 of this embodiment includes spokes 202 extendingfrom a central hub 204. The spokes 202 define a fluid flow passage 201(which may be called an airflow path) from the central hub 204 to theexterior surface 205 of the housing 162. Not shown herein, the fluidflow passage 201 would connect to a fitting, such as a brass airfitting, and a first end of a hose. The second end of the hose would beconnected to, for example, the tire stem to provide a fluid (air) flowpath from the axle to the tire. The housing 162, spokes 202, and centralhub 204 define a plurality of lubrication flow passages 207 (best seenin FIG. 4). The lubrication flow passages 207 provide a fluid path forthe lubrication from the lubrication cavity 209, which is defined by theinterior space of the hubcap 102 and a vehicle side 213 of the housing162, to the sight window lubrication cavity 211 (best seen in FIG. 3),which is defined by the sight window 104 and the sight window side 215of the housing 162. The lubrication flow passages 207 allow lubricationfluid to flow from the lubrication cavity 209 to the sight window cavity211 such that the lubrication level may be observed in the sight window104. In other words, when the vehicle is not in operation (i.e., thewheels are not rotating), the lubrication fluid can freely pass to thesight window lubrication cavity 211 so the lubrication level of thelubrication cavity 209 can be checked with the sight window 104 in thenormal course.

Additionally, the lubrication flow passages 207 provide a fluidcommunication path to the vent 105. Thus, if pressure builds in thelubrication area, the lubrication flow passages 207 allow the excessivepressure to vent from the lubrication cavity 209 through the lubricationflow passages 207 and the sight window lubrication cavity 211 and outthe vent 105. As will be explained further below, the lubrication cavity209, lubrication flow passages 207, the sight window lubrication cavity211, and the vent 105 are sealed or isolated from the air conduitssupplying pressurized air to the wheels.

With reference again to the embodiment of FIG. 1, the monitoring system150 may be operably connected to the RAU assembly 100. One exemplarymonitoring system 150 is BatRF®, which is available from STEMCO LP. Themonitoring system 150 may be a separate unit mounted to the hubcap 102,such as with the flange 151 bolted to the hubcap 102 (the bolts are notshown). Alternatively, the monitoring system 150 may be integrated in ormonolithic with the RAU assembly 100. In the exemplary embodiment shown,where the RAU assembly 100 and the monitoring system 150 are separateunits, the RAU assembly includes one or more service ports 152, such asthe two service ports 152 used in the exemplary embodiment shown inFIG. 1. The service ports 152 are connected to one or more pressureports 154 of the monitoring system through a fluid conduit 155. Only onefluid conduit 155 is shown for clarity. The fluid conduit 155 mayinclude, for example, a flexible tube along with appropriate fittings.The monitoring system 150 maintains this overall system function. Whenused, the monitoring system 150 has service ports 156.

As can be appreciated with reference to FIG. 2, the RAU assembly 100 andsight window 104 include bores 221 that align with each other and bores223 in the hubcap 102. Bolts 225 are used to couple the sight window 104and the housing 162 to the hubcap 102 in a conventional manner. As cannow be appreciated, the RAU assembly 100 and monitoring system 150provide for installation with existing parts and maintains currentfeatures and durability of existing hubcaps and seals.

FIG. 3 provides a cross-sectional view of the RAU assembly 100, thesight window 104, and the hubcap 102 in some additional detail. As canbe seen in FIG. 3, the central hub 204 of the RAU assembly 100 includesa face seal 301 and a face seal retaining ring 303, and a bearingretaining ring 307. The face seal 301, face seal retaining ring 303, andbearing retaining ring 307 all cooperate to couple a flexible torquetransfer shaft subassembly 550, described further below, and bearing 309to the RAU assembly 100. The flexible torque transfer shaft 302, whichwill be explained further below with reference to FIG. 5, has astationary shaft 304 that extends into the bearing 309. As can beappreciated, the hubcap 102, the RAU assembly 100, and the sight window104 rotate with the wheel. The stationary shaft 304, however, isstationary and is not intended to rotate with the wheel. The rotationputs a torsional force on the stationary shaft 304. The flexible torquetransfer shaft 302, however, has at least one air conduit 306 and aplurality of air fittings 308 that should not be subjected to torsionalforce if possible. The flexible torque transfer shaft 302, incombination with an axle plug 300, reduces the torsional forces on atleast the air conduit 306 and the air fittings 308 (further definedbelow in connection with FIG. 5) as will be apparent below.

As mentioned above, and with reference to FIGS. 3 and 4, the lubricationcavity 209, lubrication flow passages 207, the sight window lubricationcavity 211, and the vent 105 are sealed from the air conduits supplyingpressurized air to the wheels. To facilitate the isolation between thelubrication areas and the pressurized air areas, the present applicationprovides an air vent 400 along a likely leak path. In particular, airmay leak past, for example, the face seal 301 and the float seal 305.Any leakage may be contained in an air gap 402 between the bearing 309and the vehicle side 213 of the housing 162. The air gap 402 is coupledto the air vent 400 by a vent passage 404. The air vent 400 includes acheck valve and a protective fitting (shown but not specificallynumbered). The check valve has a very low cracking pressure to avoidpressure build up in the RAU assembly 100 and to limit the ability formoisture to enter through the air vent 400.

As best shown in FIG. 4, the left and right fluid flow passages 201allow air to flow from the central hub 204 through spokes 202. Left andright fluid flow passages 201 refer to the location on the drawing forconvenience. As is clear the device rotates and the orientation of thefluid flow passages 201 rotates with the device. In any event, a pair ofcheck valves 406 may be provided to isolate the fluid flow passages 201of each tire from the other tire. The check valves 406 may be, forexample, ball check valves including a ball 408 seated in a valve seat410 by a spring 412.

With reference now to FIG. 5, the flexible torque transfer shaft 302will be described in more detail. For ease of reference, the flexibletorque transfer shaft 302 will be explained from a RAU side 501 to anaxle side 503. The RAU side 501 is downstream and the axle side 503 isupstream.

At the RAU side 501, the float seal 315 and a float seal bushing 500 arecoupled to the stationary shaft 304 by a press fit connection or thelike. As explained in more detail above, the face seal 301 cooperateswith other elements to help retain the stationary shaft 304 in thebearing 309 and the RAU assembly 100. The bearing 309 allows the wheel(and the associated parts) to rotate while the stationary shaft 304 doesnot rotate. A float spring 509 provides a sealing force to the floatseal 305, a float seal O-ring 505, and a float seal washer 507. Thefloat seal spring 509 applies the sealing force, through the float sealwasher 507 and O-ring 505, to the float seal 301 to inhibit air fromleaking from the air supply to the air gap 402, which is shown in FIG.3.

Moving upstream from the stationary shaft 304 is a first air fitting502. The first air fitting provides an airtight connection between thestationary shaft 304 and the air conduit 306. The air conduit traversesthe flexible torque transfer shaft 302 to a second air fitting 504 atthe axle side 503. The flexible torque transfer shaft 302 is formed byan elastomeric overmolding of a portion of the stationary shaft 304, thefirst air fitting 502, the air conduit 306, and a portion of the secondair fitting 504. The flexible torque transfer shaft 302 inhibits therotation of the stationary shaft 304 and transfers the torque along itslength. A perspective view of the flexible torque transfer shaft 302 isshown in FIG. 6 from the RAU side 501 to the axle side 503. The upstreamor axle side 503 of the flexible torque transfer shaft 302 may include ataper 600 or be beveled. The taper 600 may facilitate inserting theflexible torque transfer shaft 302 into the axle plug 300 (shown in FIG.3 and described further below with reference to FIG. 8). As shown, theflexible torque transfer shaft 302 may be molded with channels 602,which may be categorized as cuts, perforations, or the like. Thechannels 602 increase the flexibility of the flexible torque transfershaft. The elastomer forms a seal between the flexible torque transfershaft 302 and the axle plug 300.

The outer surface 604 of the flexible torque transfer shaft may bemolded to have a hexagonal shape, which includes a plurality of flatsurfaces 606. The outer surface 604 could be molded with other polygonalor non-round shapes. The flat surfaces 606 are shaped to cooperativelyengage the axle plug 300, which will be explained further below. Theflat surfaces 606 when engaged with the axle plug 300 help inhibitrotation of the flexible torque transfer shaft 302. Further, withreference to FIG. 7, the stationary shaft 304 may include a lockinginterface 700 (shown in FIG. 5 as well). The locking interface 700includes one or more flat surfaces 702. The flat surfaces 702 inhibitrotation of the stationary shaft 304 once the flexible torque transfershaft 302 is molded over the locking interface 700.

With reference to FIG. 8, the axle plug 300 will now be described. Theaxle plug 300 may be molded from an elastomer similar to the flexibletorque transfer shaft 302. The elastomer may be the same or a differentelastomer as long as they are sufficiently compatible to form a sealwhen the flexible torque transfer shaft 302 is coupled to the axle plug300. As shown, the axle plug 300 includes an outer portion 802 thatincludes a steel tube 803 overmolded with an elastomer. The axle plug300 also includes a hub 804 connected to the outer portion 802 by a webof elastomer 807 that allows the hub 804 to flex for misalignmentbetween the torque transfer shaft 302 and the axle plug 300. The outersurface 810 of the outer portion 802 may include a plurality ofprotrusions 812. The protrusions 812 facilitate the seal between theaxle and the axle plug 300 as well as facilitate the insertion of theaxle plug 300 into the axle bore. The axle hub 804 has at least one rib814 extending radially inward from the inner surface 816 of the hub 804.The rib 814, in this exemplary embodiment, has two sloped surfaces 818converging to an apex 820. The sloped surfaces 818 facilitate alignmentof the flexible torque transfer shaft 302 as the flexible torquetransfer shaft 302 is moved through the bore 808 formed by the axleplug. The inner surface 818 and the rib 814 define the bore 808. Thebore 808 is shaped to cooperatively engage the flexible torque transfershaft 302. In this exemplary embodiment, the inner surface 818 forms ahexagonal shape similar to the outer surface 604. The hexagonal shapeinhibits relative rotation between the axle plug 300 and the flexibletorque transfer shaft 302.

The torque transfer shaft 302 slidingly engages the axle plug 300 toaccommodate various axle/hubcap combinations. The elastomers may bereinforced with fibers, metals, or a combination thereof to providestrength as necessary. Additionally, the elastomer should provide forflexibility (to accommodate misalignment), strength (for torquetransmission), and durability or corrosion resistance (for exposure tochemicals and heat).

With reference to FIG. 9, a cross-sectional view of the RAU assembly 100and flexible torque transfer shaft 302 is shown. As best shown in FIG.9, the RAU housing 162 includes a recess 900 to fit the sight window 104(not shown in FIG. 9). With reference to FIG. 10, the RAU assembly 100and flexible torque transfer shaft 302 are shown as installed in a wheelend. As can be appreciated, the RAU assembly 100 is coupled to thehubcap 102. The hubcap 102 is mounted to the wheel end 3 of a tire thatis mounted on a stationary, hollow axle 4. An axle spindle 5 provides acommon rotary connection, which uses wheel end bearings 6, between thewheel end 3 and the spindle 5. The axle plug 300 extends from the axlespindle 5 to receive the flexible torque transfer shaft 302. An airconduit 8, shown in FIG. 10, extends through the stationary, hollow axle4 and couples to the air fitting 504 on the upstream end of the flexibletorque transfer shaft 302.

As explained above, and summarized here, the flexible torque transfershaft 302 and axle plug 300 may limit the application of rotationaltorque on seals, conduits, and fittings. The torque transfer shaft 302and axle plug 300 further have a slideable engagement that allows useacross different axle/hubcap configurations with similar parts. In otherwords, a single combination of the RAU assembly 100, the flexible torqueshaft 302, and the axle plug 300 may be used with multiple hubcaps. Theflexible axle plug 300 and the flexible torque transfer shaft 302 each,and in combination, accommodate misalignment between the axle and theRAU 100. The flexible torque transfer shaft 302 provides a non-round andflexible shaft. The flexible torque transfer shaft subassembly 550provides an airtight high pressure air conduit 306 through thelubrication area of the hubcap.

It should be noted that the methods, systems and devices discussed aboveare intended merely to be examples. It must be stressed that variousembodiments may omit, substitute, or add various components asappropriate. Also, features described with respect to certainembodiments may be combined in various other embodiments. Differentaspects and elements of the embodiments may be combined in a similarmanner. Also, it should be emphasized that technology evolves and, thus,many of the elements are exemplary in nature and should not beinterpreted to limit the scope of the invention.

Specific details are given in the description to provide a thoroughunderstanding of the embodiments. However, it will be understood by oneof ordinary skill in the art that the embodiments may be practicedwithout these specific details.

Having described several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of theinvention. For example, the above elements may merely be a component ofa larger system. Accordingly, the above description should not be takenas limiting the scope of the invention.

We claim:
 1. A central tire inflation system for maintaining tireinflation on a vehicle, the central tire inflation system comprising: anaxle plug shaped to cooperatively engage a hollow axle of a vehicle, theaxle plug having a bore; a flexible torque transfer shaft having anupstream end shaped to cooperatively engage the bore of the axle plugand a downstream end opposite the upstream end, the flexible torquetransfer shaft comprising an air conduit extending from the upstream endto the downstream end and an overmolding; a rotary air union assemblycomprising a stationary shaft and a housing rotatably coupled to thestationary shaft, wherein the flexible torque transfer shaft overmoldingcouples the flexible torque transfer shaft and the stationary shaft andresists the rotation of the housing; and a fluid flow path contained inthe housing of the rotary air union and in fluid communication with theair conduit.
 2. The central tire inflation system of claim 1 wherein theaxle plug comprises an overmolded cylindrical tube outer portion.
 3. Thecentral tire inflation system of claim 2 wherein the axle plug furthercomprises a hub having an inner surface defining the bore and a webconnecting the hub to the overmolded cylindrical tube outer portion,wherein the web allows the hub to move with respect to the overmoldedcylindrical tube outer portion.
 4. The central tire inflation system ofclaim 3, wherein the inner surface comprises a non-round shape and theflexible torque transfer shaft has an outer surface shaped tocooperatively engage the non-round shape such that the non-round shapesresist rotation.
 5. The central tire inflation system of claim 4 whereinthe non-round shape is a polygon.
 6. The central tire inflation systemof claim 3 wherein the inner surface comprises a rib.
 7. The centraltire inflation system of claim 6 wherein the rib comprises at least twosurfaces that extend inwardly from the inner surface and converge at anapex.
 8. The central tire inflation system of claim 1 wherein theflexible torque transfer shaft overmolding comprises a plurality ofchannels.
 9. The central tire inflation system of claim 1 wherein thefluid flow path comprises a first fluid flow path to a first tire and asecond fluid flow path to a second tire.
 10. The central tire inflationsystem of claim 9, further comprising at least a third fluid flow pathto a third tire.
 11. The central tire inflation system of claim 9wherein the first fluid flow path and the second fluid flow path areisolated from each other by a plurality of check valves.
 12. The centraltire inflation system of claim 1 wherein the housing comprises aplurality of spokes extending inwardly from the housing that are coupledto a central hub and the stationary shaft extends from the central hub.13. The central tire inflation system of claim 12 wherein the housingcomprises an air vent and at least one of the plurality of spokesdefines an air vent passage.
 14. An apparatus configured for coupling toa hubcap and a hollow axle to place a fluid source in communication witha tire, the apparatus comprising: a rotary union assembly to couple to ahubcap of a vehicle, the rotary union assembly comprising: a housing tocouple to the hubcap, a plurality of spokes formed with the housing andextending radially inwardly from the housing, a central hub coupled tothe plurality of spokes, a stationary shaft coupled to and extendingfrom the central hub in a direction toward the hollow axle, and a fluidflow passage extending through the stationary shaft into the central huband through at least one of the plurality of spokes to an outer surfaceof the housing, wherein the housing, the rotary union assembly comprisesat least one lubrication flow passage formed by the housing, theplurality of spokes, and the central hub wherein the lubrication flowpassage is isolated from the fluid flow passage; and a flexible torquetransfer shaft, the flexible torque transfer shaft coupled to thestationary shaft and comprising: an air conduit having an upstream sideand a downstream side, the downstream side in fluid communication withthe fluid flow passage, and an elastomer overmold covering the airconduit and at least a portion of the stationary shaft, wherein theelastomer overmold resists rotation of the stationary shaft.
 15. Theapparatus of claim 14, further comprising an axle plug sized tocooperatively engage the hollow axle, the axle plug defining a boreshape such that the flexible torque transfer shaft slidably engages thebore.
 16. A vehicle having at least one wheel and a central tireinflation system comprising: a hubcap defining a lubrication cavity; asight window defining a sight glass lubrication cavity, the sight windowcomprising a lubrication cavity vent; a rotary union assembly residingbetween and coupled to the hubcap and the sight window, the rotary unionassembly having a housing defining an outer circumference and aplurality of spokes extending radially inward from the housing to acentral hub, the rotary union comprising a plurality of lubrication flowpassages formed by the housing, plurality of spokes, and central hubsuch that the lubrication cavity and the sight window lubrication cavityare in fluid communication; a flexible torque transfer shaft having arotary union assembly side coupled to the rotary union assembly and anaxle side opposite the rotary union assembly side, the flexible torquetransfer shaft comprising an air conduit; an axle plug to cooperativelyengage an axle of the vehicle and a bore sized to slidingly engage theflexible torque transfer shaft; and an air path is contained in the airconduit and the rotary union assembly to place an air source in fluidcommunication with a tire.
 17. The vehicle of claim 16 furthercomprising a tire pressure monitoring system in fluid communication withthe air path.
 18. The vehicle of claim 17 wherein the rotary unionassembly comprises at least one service port and the tire pressuremonitoring system comprises at least one pressure port in fluidcommunication with the at least one service port.
 19. The vehicle ofclaim 17 wherein the tire pressure monitoring system is monolithic withthe rotary union assembly.
 20. A vehicle having at least one wheel and acentral tire inflation system comprising: a hubcap defining alubrication cavity; a sight window defining a sight glass lubricationcavity, the sight window comprising a lubrication cavity vent; apressure monitor coupled to the hubcap; a rotary union assembly residingbetween and coupled to the hubcap and the sight window, the rotary unionassembly having a housing defining an outer circumference and aplurality of spokes extending radially inward from the housing to acentral hub, the rotary union comprising a plurality of lubrication flowpassages formed by the housing, plurality of spokes, and central hubsuch that the lubrication cavity and the sight window lubrication cavityare in fluid communication; a flexible torque transfer shaft having arotary union assembly side coupled to the rotary union assembly and anaxle side opposite the rotary union assembly side, the flexible torquetransfer shaft comprising an air conduit; an axle plug to cooperativelyengage an axle of the vehicle and a bore sized to slidingly engage theflexible torque transfer shaft; and an air path is contained in the airconduit and the rotary union assembly to place an air source in fluidcommunication with a tire and the pressure monitor.
 21. The vehicle ofclaim 20 wherein the pressure monitor is integrated into the rotaryunion assembly.